I. Field of the Invention
This invention relates to power amplifiers. More particularly, the present invention is directed to a power amplifier using backgate terminals of non-linear amplifier stages as a substitute for switching transistors.
II. Description of the Related Art
Many modulation methods used in wireless communications employ phase and amplitude variation to increase the spectral efficiency of an RF signal. Known power amplifiers used for modulation in transmitters of communication systems must operate linearly so that phase and amplitude information in a transmitted signal are preserved. One drawback of using linear amplifiers for this purpose lies in the amplifier's low efficiency in converting battery power, used in many communication system applications to power the amplifier, into signal power.
One technique for overcoming the low conversion efficiency of linear amplifiers is to employ an array of switchable high efficiency nonlinear amplifiers as a substitute for a linear amplifier. Such a technique is depicted in the prior art configuration of FIG. 1 and its schematic equivalent shown in FIG. 2. As illustrated in these figures, a plurality of nonlinear amplifier stages X.sub.1 -X.sub.n (which may be equivalent or may output different power from each other) are connected in parallel and are used to amplify an input Rf signal (Rf.sub.in) to produce an output Rf signal (Rf.sub.out). Each amplifier stage is controlled by a separate clock signal (C.sub.1 -C.sub.n).
As shown in FIG. 2, each amplifier stage is implemented using a transistor, such as a MOSFET (Q.sub.1, Q.sub.3, . . . Q.sub.n, having a gate (G), a source (S), and a drain (D) terminal. Each such transistor is controlled by a corresponding switch transistor (Q.sub.2, Q.sub.4 . . . Q.sub.n+1) which, in turn, is activated by a corresponding clock signal C.sub.1, C.sub.3 . . . C.sub.n. Thus, when C.sub.1 is a certain value (e.g. a logic "1" ), Q.sub.2 will turn on and thereby activate Q.sub.1, etc., causing a current to flow across a load "L" and generating a power associated therewith. A drawback of the circuit of FIG. 2 is that the switching transistors have significant on-resistance that consumes power and reduces the overall operating efficiency of the amplifier.